Organic light emitting device comprising multilayer cathode

ABSTRACT

An organic light emitting device that includes a cathode, an anode and an organic layer arranged between the cathode and the anode, wherein the cathode includes at least one metal layer and at least one inorganic electrode layer alternately arranged, the cathode includes at least three layers. The organic light emitting device has excellent luminous efficiency, luminance, color coordinate characteristic, power efficiency and an increased lifetime while preventing the cathode electrode from diffusing into the organic emitting layer.

CLAIM OF PRIORITY

This application makes reference to, incorporates the same herein, andclaims all benefits accruing under 35 U.S.C.§119 from an application forORGANIC ELECTROLUMINESCENCE DEVICE COMPRISING MULTILAYER CATHODE earlierfiled in the Korean Intellectual Property Office on 16 Feb. 2005 andthere duly assigned Serial No. 10-2005-0012913.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an organic light emitting device, andmore particularly, to an organic light emitting device which hasexcellent luminous efficiency, luminance, color coordinatecharacteristic, power efficiency and an increased lifetime by using amultilayered cathode.

2. Description of the Related Art

Organic light emitting devices (OLEDs) are self-emissive devices using aphenomenon that when electrical current is applied to a fluorescent orphosphorescent organic layer, electrons and holes are combined in theorganic layer to emit light. OLEDs have advantages such as being lightin weight, having simple constitutional elements, easy to fabricate,having high image quality and having high color purity. Further, OLEDscan realize moving pictures perfectly and can be operated at low powerconsumption. Thus, vigorous research is being conducted on OLEDs.

An OLED has an anode electrode layered on a substrate and an holeinjection layer (HIL) and an hole transport layer (HTL) as hole-relatedlayers sequentially layered on the anode electrode, an emitting layer(EML) layered on the HTL, and a cathode electrode layered on the EML.

Many attempts have been made to increase luminous efficiency or powerefficiency and lifetime of the OLED by improving characteristics of thecathode electrode. With regard to this, cathode electrodes having amultilayer structure or having various interlayers have been suggested.

For example, U.S. Pat. No. 6,255,774 B1 to Pichler describes amultilayer cathode having a first electrode having a work function of3.7 eV or less and a thickness of 5 nm or less and a second electrode.U.S. Pat. Nos. 6,558,817, 5,739,635, and 6,541,790 B1 and EP 1,336,995A2 describe cathode electrodes including various interlayers. However,when devices are driven while controlling injection of electricalcurrent into these cathodes, the cathode electrodes diffuse into EMLswith time. As a result, the luminous efficiency and lifetime of thedevices decrease. What is therefore needed is an improved design for anOLED using a multi layered cathode that limits or prevents the cathodeelectrode from diffusing into the EMLs.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide animproved design for an organic light emitting device.

It is further an object of the present invention to provide an improveddesign for a multi layered cathode electrode in an OLED.

It is yet an object of the present invention to provide a design for anOLED that includes a multi layered cathode where, over time, the cathodeelectrode does not diffuse into the emitting layers of the device.

It is still an object of the present invention to provide an OLED havingexcellent luminous efficiency, improved luminance, improved colorcoordinate characteristic, improved power efficiency and an increasedlifetime by controlling injection of electrical current into a cathodeelectrode and preventing diffusion of the cathode electrode into anemitting layer (EML).

According to an aspect of the present invention, there is provided anOLED that includes a cathode, an anode and an organic layer arrangedbetween the cathode and the anode, wherein the cathode includes at leastone metal layer and at least one inorganic electrode layer alternatelyarranged, the cathode having at least three layers.

The at least one metal layer can have a work function of 2.0-7.0 eV. Theat least one metal layer can be one or more of Li, Cs, Ca, Ba, Mg, Al,Ag and Au. The at least one metal layer can have a thickness between 0.2and 500 nm. The at least one inorganic electrode layer can include oneor more of a metal oxide, a metal halide, a metal nitride and a metalperoxide. The at least one inorganic electrode layer can include one ofBaF₂, LiF, CsF, BaF₂, MgF₂, Al₂O₃, MgO, and LiO₂. The at least oneinorganic electrode layer can have a thickness between 0.1 and 30 nm.

The cathode can be a stacked structure in which one of said at least oneinorganic electrode layers is arranged on the organic layer, one of saidat least one metal layer is arranged on the one of said at least oneinorganic electrode layer, and another of said at least one inorganicelectrode layer is arranged on the one of the at least one metal layer.

The cathode can be a stacked structure of at least four layers, thecathode including one of the at least one inorganic electrode layerarranged on the organic layer, one of said at least one metal layerarranged on the one of the at least one inorganic electrode layer,another of the at least one inorganic electrode layer is arranged on theone of the at least one metal layer, and another of the at least onemetal layer is arranged on the another of the at least one inorganicelectrode layer.

The multilayer cathode can have a stacked structure of at least fourlayers where one of the at least one inorganic electrode layer isarranged on the organic layer, one of the at least one metal layer isarranged on the one of the at least one inorganic electrode layer,another of the at least one inorganic electrode layer is arranged on theone of the at least one metal layer, and another of the at least onemetal layer is arranged on the another of the at least one inorganicelectrode layer.

The one of the at least one metal layer can have a lower work functionthan that of the another of the at least one metal layer. The one of theat least one metal layer can have a work function of 3.5 eV or less. Theone of the at least one metal layer can have a thickness between 0.2 and100 nm and the another of the at least one metal layer has a thicknessbetween 5 and 500 nm.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof, will be readily apparent as the same becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings in which likereference symbols indicate the same or similar components, wherein:

FIG. 1 is a schematic cross-sectional view of an OLED;

FIG. 2 is a schematic cross-sectional view of an OLED having amultilayer cathode;

FIG. 3 is a schematic cross-sectional view of an OLED according to anembodiment of the present invention;

FIG. 4 is a graph of luminance vs. luminous efficiency for an OLEDaccording to an embodiment of the present invention together with thatof another OLED;

FIG. 5 is a graph of current density vs. luminous efficiency for an OLEDaccording to an embodiment of the present invention together with thatof another OLED;

FIG. 6 is a graph of current density vs. power efficiency for an OLEDaccording to an embodiment of the present invention together with thatof another OLED; and

FIG. 7 is a graph of voltage vs. luminous efficiency and powerefficiency for an OLED according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the figures, FIG. 1 is a schematic cross-sectional viewof an organic light emitting device. Referring to FIG. 1, the organiclight emitting device (OLED) has a structure in which an anode electrode12 is layered on a substrate 11 and a hole injection layer (HIL) 13 anda hole transport layer (HTL) 14 as hole-related layers are sequentiallylayered on the anode electrode 12, an emitting layer (EML) 15 is layeredon the HTL 14, and a cathode electrode 16 is layered on the EML 15.

Many attempts have been made to increase luminous efficiency or powerefficiency and lifetime of the OLED by improving characteristics of thecathode electrode 16. With regard to this, cathode electrodes can be amultilayered structure or can have various interlayers.

For example, FIG. 2 illustrates an OLED having a multilayer cathode. TheOLED of FIG. 2 includes a substrate 21, an anode 22 located on thesubstrate 21, an HIL 23 located on anode 22, an HTL 24 on HIL 23, an EML25 on HTL 24 and the multi-layered cathode on the EML 25. In FIG. 2, thecathode includes a first electrode 26 having a work function of 3.7 eVor less and a thickness of 5 nm or less and a second electrode 27 on topof the first electrode 26. However, when such a device is driven whilecontrolling injection of electrical current into the cathode, thecathode electrode diffuse into EMLs with time. Thus, the luminousefficiency and lifetime of the device of FIG. 2 decreases.

Turning now to FIG. 3, FIG. 3 is a schematic cross-sectional view of anOLED according to an embodiment of the present invention. Referring toFIG. 3, the cathode of the OLED has a stacked structure of at leastthree layers (four shown in FIG. 3). In FIG. 3, the cathode is shown tohave an inorganic electrode layer 36, a first metal layer 37, a secondinorganic electrode layer 38, and a second metal layer 39 aresequentially stacked upon one another. The first inorganic electrodelayer 36 functions as an electron injection layer and the secondinorganic electrode layer 38 prevents diffusion of the second metallayer 39 into an emitting layer 35. Reference numerals 31, 32, 33, 34,and 35 respectively denote an anode electrode, an HIL, an HTL, anemission region, and an emitting layer.

In the OLED of FIG. 2, the cathode metal layer diffuses into an organiclayer, thus decreasing luminous efficiency and lifetime of the device.An OLED according to an embodiment of the present invention includes amulti layered cathode, where at least one layer of the cathode is aninorganic electrode layer that serves to prevent the diffusion of acathode metal electrode into an organic layer. In other words, theinorganic electrode layer of a cathode electrode functions as a barrierthat prevents diffusion.

The cathode metal layer used in an embodiment of the present inventionmay be made of a metal having a work function between 2.0 and 7.0 eV.There is no suitable metal that has a work function less than 2.0 eV,and if there were any, such a metal would induce an excess injection ofelectrons and would be unstable to oxygen or moisture. Similarly, thereis no suitable metal having a work function greater than 7.0 eV, and ifthere were any, such a metal would have too high a work function andthus, electrons could not be easily injected into the metal layer.Examples of metals having a work function between 2.0 and 7.0 eVinclude, but are not limited to, Li, Cs, Ca, Ba, Mg, Al, Ag, Au, andalloys thereof.

The metal layer may have a thickness between 0.2 and 500 nm. If thethickness of the metal layer is less than 0.2 nm, the metal layer cannotfunction as an electrode. If the thickness of the metal layer is greaterthan 500 nm, the characteristics of the device are not further improved.

The inorganic electrode layer used in an embodiment of the presentinvention maybe made of a metal oxide, a metal halide, a metal nitride,a metal peroxide or a mixture thereof. Specific examples of materialsthat can be used in the inorganic electrode layer include, but are notlimited to LiF, CsF, BaF₂, MgF₂, Al₂O₃, MgO, LiO₂, and mixtures thereof.

The inorganic electrode layer may have a thickness between 0.1 and 30nm. If the thickness of the inorganic electrode layer is less than 0.1nm, the inorganic electrode layer cannot function as a diffusionbarrier. If the thickness of the inorganic electrode layer is greaterthan 30 nm, a conductivity through the inorganic electrode layerdecreases preventing the cathode from properly functioning.

Although the OLED with having a cathode having the stacked structure offour layers, i.e., in which the first inorganic electrode layer 36, thefirst metal layer 37, the second inorganic electrode layer 38, and thesecond metal layer 39 are sequentially stacked upon one another, isillustrated in FIG. 3, the present invention is not limited to thisstructure. For example, an OLED according to another embodiment of thepresent invention can have a cathode having a stacked structure of justthree layers. One example of such a three layered cathode of the presentinvention would be an inorganic electrode layer being stacked on theorganic layer, and a metal layer being stacked on the inorganicelectrode layer and a second inorganic electrode layer being stacked onthe metal layer. Another example of a three layered cathode according tothe present invention would be a metal layer on the EML, an inorganicelectrode layer on the metal layer and then a second metal layer on theinorganic electrode layer.

When the cathode has at least two inorganic electrode layers, a materialof a first inorganic electrode layer can be the same as or differentfrom that of a second inorganic electrode layer. When the device has atleast two metal layers, a material of a first metal layer can be thesame as or different from that of a second metal layer.

When the device has at least two metal layers and the material of thefirst metal layer is different from that of the second metal layer, thefirst metal layer being closer to the emitting layer than the secondmetal layer, it is preferable that the first metal layer has a lowerwork function than that of the second metal layer since the first metallayer joins to an organic layer and determines a difference in theenergy level between the metal work function and the lowest unoccupiedmolecular orbital (LUMO) of the organic layer, i.e., a barrier forelectron injection.

It is preferable that the first metal layer have a work function of 3.5eV or less, since in most cases, LUMO of an organic material has a workfunction of 3.5 eV or less and as the work function of the metaldecreases, the electrons can be easily injected into the organic layer.

The first metal layer can have a thickness between 0.2 and 100 nm andthe second metal layer can have a thickness between 5 and 500 nm. If thethickness of the first metal layer is less than 0.2 nm, the first metallayer cannot function as an electrode. If the thickness of the firstmetal layer is greater than 100 nm, a second or a third metal layercannot efficiently function as an electrode. If the thickness of thesecond metal layer is less than 5 nm, a surface conductivity decreases.If the thickness of the second metal layer is greater than 500 nm, thecharacteristics of the device are not further improved.

The structure of an OLED according to an embodiment of the presentinvention will now be described in more detail. The OLED can be producedusing a high molecular weight emitting layer (EML) or a low molecularweight EML. The OLED using the high molecular weight EML includes ananode electrode formed on a substrate, an HIL formed on the anodeelectrode, an HTL formed on the HIL, an EML formed on the HTL, anelectron transport layer (ETL) formed on the EML, an electron injectionlayer (EIL) formed on the ETL, and a cathode electrode formed on theEIL.

The substrate can be a glass substrate or a transparent plasticsubstrate, which have excellent transparency, surface smoothness, easyhandling, and excellent waterproofness.

When the OLED is a front emission type device, the anode electrodeformed on the substrate is a reflective metal layer. When the OLED is arear emission type device, the anode electrode can be made of atransparent and highly conductive material, such as indium tin oxide(ITO), indium zinc oxide (IZO), tin oxide (SnO₂), zinc oxide (ZnO), or amixture thereof.

When a high molecular weight EML is used, the HIL can have a thicknessbetween 50 and 1500 Å. If the thickness of the HIL is less than 50 Å,the hole injection property is poor. If the thickness of the HIL isgreater than 1500 Å, the hole injection ability is not further improvedand a driving voltage can increase according to a material of the HIL,which is undesirable. The HTL can have a thickness between 50 and 1500Å. If the thickness of the HTL is less than 50 Å, the hole transportproperty is poor. If the thickness of the HTL is greater than 1500 Å, adriving voltage increases, which is undesirable. In the OLED using thehigh molecular weight EML, a phosphorescent material and a fluorescentmaterial can be used in the EML. An EIL can be selectively formed on theEML. The EIL can be made of, for example ionomer (for example, sodiumsulfonated polystyrene), a metal halide (for example, LiF, CsF, orBaF₂), or a metal oxide (for example, Al₂O₃). A multilayer cathodeaccording to an embodiment of the present invention is formed on the EML(when the device does not include an EIL) or on the EIL (when the deviceincludes an EIL).

An OLED using the low molecular weight EML includes an anode electrodeformed on a substrate, an HIL formed on the anode electrode, an HTLformed on the HIL, an EML formed on the HTL, an ETL formed on the EML,an EIL formed on the ETL, and a cathode electrode formed on the EIL. TheOLED using the low molecular weight EML can use the same substrate andanode electrode as the high molecular weight OLED.

When a low molecular weight EML is used, the HIL can have a thicknessbetween 50 and 1500 Å. If the thickness of the HIL is less than 50 Å, ahole injection property is poor. If the thickness of the HIL is greaterthan 1500 Å, the driving voltage increases, which is undesirable.

When a high molecular weight EML is used, the HTL can have a thicknessof 50-1500 Å. If the thickness of the HTL is less than 50 Å, a holetransport property is poor. If the thickness of the HTL is greater than1500 Å, a hole injection ability is not further improved and the drivingvoltage can increase according to a material of the HIL, which isundesirable.

In the OLED using the low molecular weight EML, a red light-emittingmaterial in a red light region (R), a green light-emitting material in agreen light region (G), and a blue light-emitting material in a bluelight region are respectively patterned to obtain EMLs which correspondto pixel regions. Each of the light emitting materials can be a mixtureof at least two host materials.

The EML can have a thickness between 100 and 2000 Å, and preferablybetween 300 and 400 Å. If the thickness of the EML is less than 100 Å,luminous efficiency and lifetime decreases. If the thickness of the EMLis greater 2000 Å, a driving voltage increases, which is undesirable.

In the OLED using the low molecular weight EML, the ETL is formed on theEML. The ETL can be made of, for example, Alq3. The ETL can have athickness between 50 and 600 Å. If the thickness of the ETL is less than50 Å, the lifetime decreases. If the thickness of the ETL is greaterthan 600 Å, a driving voltage increases, which is undesirable.

An EIL can be selectively formed on the ETL. The EIL can be made of, forexample, LiF, NaCl, CsF, Li₂O, BaO, or Liq. The EIL can have a thicknessbetween 1 and 100 Å. If the thickness of the EIL is less than 1 Å, theEIL cannot efficiently finction, and thus a driving voltage increases.If the thickness of the EIL is greater than 100 Å, the EIL functions asan insulating layer, and thus the driving voltage increases.

Essentially, the structure of the OLED having a low molecular weight EMLis similar to an OLED having a high molecular weight EML. Both designscan include an HIL, an HTL, an EML, an ETL and an EIL. Further, themulti-layer cathode of the present invention can be applied to each ofthe high molecular weight OLED and the low molecular weight OLED.

Next, a multilayer cathode according to an embodiment of the presentinvention is formed on the ETL. In forming the cathode, an inorganicelectrode layer can not be directly formed on an EIL layer.

An OLED according to an embodiment of the present invention can bemanufactured as follows. First, a material for forming an anodeelectrode is coated on a substrate. An insulating layer (or pixel definelayer(PDL)) defining a pixel region can be formed on the anodeelectrode.

Then, an HIL, which is an organic layer, is coated on the resultantstructure. The HIL can be formed on the anode electrode using a method,such as a vacuum thermal deposition method or a spin coating method.

Then, an HTL can be optionally formed on the HIL using a vacuum thermaldeposition method or a spin coating method, etc. The EML can be formedon the HIL (when the device does not include an HTL) or on the HTL (whenthe device does include an HTL). The EML can be formed using a methodsuch as vacuum deposition, inkjet printing, laser induced thermalimaging, photolithography, etc.

Subsequently, an ETL and an EIL can be optionally formed on the EMLusing a vacuum thermal deposition method or a spin coating method. Then,a multilayer cathode according to an embodiment of the present inventionis coated on the resultant structure using a vacuum thermal depositionmethod. The resultant structure is then encapsulated.

The present invention will be described in more detail with reference tothe following examples. It is to be understood that these examples aregiven for the purpose of illustration and are not intended to limit thescope of the invention.

EXAMPLE 1

An ITO glass substrate with a surface resistance of 15 Ω/cm² (1200 Å)(available from Samsung Corning Corporation) was cut into a size of 50mm×50 mm×0.7 mm and sonicated in pure water and isopropyl alcohol,respectively, for 5 minutes and cleaned with UV light and ozone,respectively, for 30 minutes. Then, Poly(3,4-ethylenedioxythiophene)poly(styrenesulfonate) (PEDOT/PSS) (Baytron P AI4083, available fromBayer) was coated on the ITO glass substrate to a thickness of 50 nm at2,000 rpm and the coated substrate was heated at 200° C. for 10 minuteson a hot plate.

A hole transport material,poly(9,9-dioctylfluorene-co-bis-N,N′-(4-butylphenyl)-bis-N,N′-phenyl-1,4-phenylenediamine(PFB, available from Dow Chemical Company) was spin coated on theresultant HIL to form an HTL having a thickness of 10 nm. Then, theresultant product was heated at 220° C. for 1 hour under nitrogenatmosphere.

Subsequently, poly(2′,3′,6′,7′-tetraoctyloxyspirofluorene-co-N-(4′-ethylhexyloxy)-phenoxazine)(TS-9), which isapolyspirofluorene-based blue light-emitting material having a weightaverage molecular weight of 1,500,000, was dissolved in xylene in aconcentration of 1.0% by weight. The resultant solution was transferredonto an HTL using a micropipette and spin coated, and then the resultantstructure was heated at 200° C. for 30 minutes.

Then, BaF₂ was vacuum deposited to a thickness of 5 nm on the EML, andCa was vacuum deposited to a thickness of 3.3 nm on the BaF₂ layer, BaF₂was again vacuum deposited to a thickness of 0.5 nm on the Ca layer, andthen Al was vacuum deposited to a thickness of 300 nm on the BaF₂ layer.The resultant structure was encapsulated to manufacture an OLEDaccording to an embodiment of the present invention.

Comparative Example

An OLED was manufactured in the same manner as in Example 1, except thatafter BaF₂/Ca was vacuum deposited to a thickness of 5 nm on the EML, Caand Al were vacuum deposited to thicknesses of 3.3 nm and 300 nm,respectively.

Performance Test

FIG. 4 is a graph of luminance vs. luminous efficiency for an OLEDmanufactured in Example 1 and an OLED manufactured in the ComparativeExample. FIG. 5 is a graph of current density vs. luminous efficiencyfor an OLED manufactured in Example 1 and an OLED manufactured in theComparative Example. FIG. 6 is a graph of current density vs. powerefficiency for an OLED manufactured in Example 1 and an OLEDmanufactured in the Comparative Example. FIG. 7 is a graph of voltagevs. luminous efficiency and power efficiency for an OLED manufactured inExample 1.

Referring to FIG. 4, the OLED manufactured in Example 1 according to anembodiment of the present invention had higher luminous efficiency thanthe OLED manufactured in the Comparative Example, as the luminanceincreased.

Referring to FIGS. 5 and 6, the OLED manufactured in Example 1 accordingto an embodiment of the present invention had higher luminous efficiencyhigher power efficiency by 20% than the OLED made in the Comparativeexample, at a given current density.

Referring to FIG. 7, the OLED manufactured in Example 1 according to anembodiment of the present invention had high luminous efficiency andpower efficiency at a given voltage. A power efficiency of 10 lm/Wcorresponds to the highest power efficiency in an OLED using a singlelayer of blue light emitting polymer.

An OLED according to the present invention has excellent luminousefficiency, luminance, color coordinate characteristic and powerefficiency and an increased lifetime by controlling injection ofelectrical current into a cathode electrode and preventing diffusion ofthe cathode electrode into an EML.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. An organic light emitting device (OLED), comprising: a cathode; ananode; and an organic layer arranged between the cathode and the anode,wherein the cathode comprises at least one metal layer and at least oneinorganic electrode layer alternately arranged, the cathode comprisingat least three layers.
 2. The OLED of claim 1, wherein the at least onemetal layer has a work function in the range of 2.0-7.0 eV.
 3. The OLEDof claim 1, wherein the at least one metal layer comprises a materialselected from the group consisting of Li, Cs, Ca, Ba, Mg, Al, Ag, Au,and an alloy thereof.
 4. The OLED of claim 1, wherein the at least onemetal layer has a thickness in the range of 0.2-500 nm.
 5. The OLED ofclaim 1, wherein the at least one inorganic electrode layer comprises amaterial selected from the group consisting of a metal oxide, a metalhalide, a metal nitride, a metal peroxide, and a mixture thereof.
 6. TheOLED of claim 5, wherein the at least one inorganic electrode layercomprises a material selected from the group consisting of BaF₂, LiF,CsF, BaF₂, MgF₂, Al₂O₃, MgO, and LiO₂.
 7. The OLED of claim 1, whereinthe at least one inorganic electrode layer has a thickness in the rangeof 0.1-30 nm.
 8. The OLED of claim 1, wherein the cathode is a stackedstructure in which one of said at least one inorganic electrode layersis arranged on the organic layer, one of said at least one metal layeris arranged on the one of said at least one inorganic electrode layer,and another of said at least one inorganic electrode layer is arrangedon the one of the at least one metal layer.
 9. The OLED of claim 1,wherein the cathode comprises a stacked structure of at least fourlayers comprising one of the at least one inorganic electrode layerarranged on the organic layer, one of said at least one metal layerarranged on the one of the at least one inorganic electrode layer,another of the at least one inorganic electrode layer arranged on theone of the at least one metal layer, and another of the at least onemetal layer arranged on the another of the at least one inorganicelectrode layer.
 10. The OLED of claim 1, wherein the multilayer cathodehas a stacked structure of at least four layers where one of the atleast one inorganic electrode layer is arranged on the organic layer,one of the at least one metal layer is arranged on the one of the atleast one inorganic electrode layer, another of the at least oneinorganic electrode layer is arranged on the one of the at least onemetal layer, and another of the at least one metal layer is arranged onthe another of the at least one inorganic electrode layer.
 11. The OLEDof claim 9, wherein the one of the at least one metal layer has a lowerwork function than that of the another of the at least one metal layer.12. The OLED of claim 10, wherein the one of the at least one metallayer has a lower work function than that of the another of the at leastone metal layer.
 13. The OLED of claim 9, wherein the one of the atleast one metal layer has a work function of 3.5 eV or less.
 14. TheOLED of claim 10, wherein the one of the at least one metal layer has awork function of 3.5 eV or less.
 15. The OLED of claim 9, wherein theone of the at least one metal layer has a thickness in the range of0.2-100 nm and the another of the at least one metal layer has athickness in the range of 5-500 nm.
 16. The OLED of claim 10, whereinthe one of the at least one metal layer has a thickness in the range of0.2-100 nm and the another of the at least one metal layer has athickness in the range of 5-500 nm.
 17. An organic light emitting device(OLED), comprising: a cathode; an anode; and an organic layer arrangedbetween the cathode and the anode, wherein the cathode comprises atleast one metal layer and at least one inorganic electrode layeralternately arranged, one of the at least one inorganic electrode layerbeing adapted to prevent diffusion from one of the at least one metallayer into the organic layer during a lifetime of the OLED, the cathodefurther comprising at least one of another inorganic electrode layer andanother metal layer.
 18. The OLED of claim 17, the cathode comprising: afirst of the at least one inorganic electrode layer arranged on theorganic layer; a first of the at least one metal layer arranged on thefirst of the at least one inorganic electrode layer; a second of the atleast one inorganic electrode layer arranged on the first of the atleast one metal layer; and a second of the at least one metal layerarranged on the second of the at least one inorganic electrode layer.19. The OLED of claim 18, the second of the at least one inorganicelectrode layer being adapted to prevent diffusion from the second ofthe at least one metal layer into the organic layer.
 20. The OLED ofclaim 17, the cathode comprising: a first of the at least one metallayer arranged on the organic layer; a first of the at least oneinorganic electrode layer arranged on the first of the at least onemetal layer; and a second of the at least one metal layer arranged onthe first of the at least one inorganic electrode layer, the first ofthe at least one inorganic electrode layer being adapted to preventdiffusion from the second of the at least one metal layer into theorganic layer.